A bearing is a device that permits constrained relative motion between two components. Bearings may be used in many different types of machinery to retain and support rotating components such as, for example, a wheel on a vehicle, a vane on a windmill or a drum in a washing machine. A rolling element bearing comprises inner and outer rings and a plurality of rolling elements (balls or rollers). As the bearing rotates, the raceways of the inner ring and outer ring make contact with the rolling elements. This results in a wear path on both the rolling elements and the raceways. For a variety of reasons it sometimes happens that a bearing does not attain its calculated rating life. Rolling fatigue can, for instance, occur as a result of wear and fretting.
The minimization of wear and fretting is therefore important to extend the fatigue life of bearings. One way of extending the fatigue life of a bearing is to apply lubrication which, by means of an oil film, reduces the friction and wear in the contact areas of the respective bearing components.
It is further known to try to reduce wear by using, for example, a hard coating. However, these hard coatings have tended to be ceramic, which are brittle in nature and therefore subject to cracking and breaking up, thereby forming debris. This debris in turn results in an increase in wear. Therefore, these coatings, instead of alleviating the effects of wear and fretting, can actually increase the wear and fretting under some circumstances.
Currently, the most successful approach to countering the effects of fretting and wear on bearings is to use a lubricating oil, grease or paste which includes one or more additives with anti-wear and/or anti-fretting properties to reduce the friction between bearing components. Over time the additives form a layer that prevents metal-to-metal contact between the rolling elements and the raceway. A disadvantage of these additives is that a chemically active compound is needed, to provide a certain acidity for a chemical reaction. The chemical reaction takes place between the metal matrix and the active compound, which deteriorates the metal matrix. Further disadvantages of such applications are the facts that the layer is not present from the start of the use of the bearing and that formation of the layer only occurs when a particular temperature has been reached.
In JP 3271378 the formation of a protective layer on the surface of a sliding member or rolling member in the dynamic state has been described. The layer is formed by means of a tribochemical reaction in which reaction a chemically active compound such as an organic sulphur compound, organic phosphorus compound or an organic chlorine compound is used. The use of such compounds means that the contact surfaces are chemically attacked which may result in corrosion, fretting and wear problems.
Further, in WO 2009/071674 A2, the curved surface of a low-friction mechanical element is covered with a layer of tungsten disulphide by way of a tribochemical deposition process during which process the tungsten disulphide is formed as a result of the chemical reaction between a tool having a tungsten-carbide surface and a process fluid that contains sulphur.